When decoding convolutionally encoded codewords in a blind manner, a receiver does not know where a codeword starts and/or ends. For this reason, the decoder may try to decode fragments of codewords, concatenations of fragments of distinct codewords, or even concatenations of noise and fragments of codewords. This is referred to as desynchronization of a trellis, and results in decoding problems for both windowed decoding of terminated CC codewords, and iterative decoding of Tail-Biting Convolutionally Coded (TBCC) codewords. Continuing decoding of a desynchronized trellis wastes power and adds processing delay.
Desynchronization of a trellis is relevant to Long Term Evolution (LTE) Physical Downlink Control Channel (PDCCH) blind detection, but also applies to other non-LTE applications of convolutional coding, such as non-LTE cellular/wireless communication, wireline communication, and data storage access. The decoder and decoding method aspects described herein are applicable to any situation where the decoder does not know where the codeword begins and/or ends.
In LTE PDCCH, a User Equipment (UE) receiver attempts TBCC decoding of multiple PDCCH candidates, blindly, until a correctly decoded TBCC codeword directed to the specific UE is found by means of cyclic redundancy check (CRC) match. A PDCCH candidate may be determined by three parameters: codeword size (CS), aggregation level (AL), and location (L) in the control region of the OFDMA subframe. During this blind detection process, a number of different candidates may be attempted to be decoded, at times this results in power-wastage and increased processing latency.
Because in LTE a UE cannot extract useful downlink user traffic until the UE has completed the PDCCH blind detection, a latency increase in the blind detection then incurs in an indirect increase in power and complexity. As PDCCH detection time increases, the number of full Orthogonal Frequency-Division Multiple Access (OFDMA) symbols that need to be fully buffered for potential subsequent processing increases. The Fast Fourier Transform (FFT) processing on OFDM symbols needs to be full, as the UE does not know yet in which subcarriers its allocations lie. When no allocations exist for the UE, the amount of time it takes for the UE to reach a power-saving state will increase. When allocations exist for the UE, the amount of time available for user traffic processing will decrease, thereby requiring more compute power to finish in time. For low-power LTE devices, it is desired to accelerate the PDCCH blind detection procedure.